clippy_lints/src/bytecount.rs

   1 use crate::utils::{
   2     contains_name, get_pat_name, match_type, paths, single_segment_path, snippet_with_applicability,
   3     span_lint_and_sugg, walk_ptrs_ty,
   4 };
   5 use if_chain::if_chain;
   6 use rustc::declare_lint_pass;
   7 use rustc::hir::*;
   8 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   9 use rustc::ty;
  10 use rustc_errors::Applicability;
  11 use rustc_session::declare_tool_lint;
  12 use syntax::ast::{Name, UintTy};
  13
  14 declare_clippy_lint! {
  15     /// **What it does:** Checks for naive byte counts
  16     ///
  17     /// **Why is this bad?** The [`bytecount`](https://crates.io/crates/bytecount)
  18     /// crate has methods to count your bytes faster, especially for large slices.
  19     ///
  20     /// **Known problems:** If you have predominantly small slices, the
  21     /// `bytecount::count(..)` method may actually be slower. However, if you can
  22     /// ensure that less than 2³²-1 matches arise, the `naive_count_32(..)` can be
  23     /// faster in those cases.
  24     ///
  25     /// **Example:**
  26     ///
  27     /// ```rust
  28     /// # let vec = vec![1_u8];
  29     /// &vec.iter().filter(|x| **x == 0u8).count(); // use bytecount::count instead
  30     /// ```
  31     pub NAIVE_BYTECOUNT,
  32     perf,
  33     "use of naive `<slice>.filter(|&x| x == y).count()` to count byte values"
  34 }
  35
  36 declare_lint_pass!(ByteCount => [NAIVE_BYTECOUNT]);
  37
  38 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ByteCount {
  39     fn check_expr(&mut self, cx: &LateContext<'_, '_>, expr: &Expr<'_>) {
  40         if_chain! {
  41             if let ExprKind::MethodCall(ref count, _, ref count_args) = expr.kind;
  42             if count.ident.name == sym!(count);
  43             if count_args.len() == 1;
  44             if let ExprKind::MethodCall(ref filter, _, ref filter_args) = count_args[0].kind;
  45             if filter.ident.name == sym!(filter);
  46             if filter_args.len() == 2;
  47             if let ExprKind::Closure(_, _, body_id, _, _) = filter_args[1].kind;
  48             then {
  49                 let body = cx.tcx.hir().body(body_id);
  50                 if_chain! {
  51                     if body.params.len() == 1;
  52                     if let Some(argname) = get_pat_name(&body.params[0].pat);
  53                     if let ExprKind::Binary(ref op, ref l, ref r) = body.value.kind;
  54                     if op.node == BinOpKind::Eq;
  55                     if match_type(cx,
  56                                walk_ptrs_ty(cx.tables.expr_ty(&filter_args[0])),
  57                                &paths::SLICE_ITER);
  58                     then {
  59                         let needle = match get_path_name(l) {
  60                             Some(name) if check_arg(name, argname, r) => r,
  61                             _ => match get_path_name(r) {
  62                                 Some(name) if check_arg(name, argname, l) => l,
  63                                 _ => { return; }
  64                             }
  65                         };
  66                         if ty::Uint(UintTy::U8) != walk_ptrs_ty(cx.tables.expr_ty(needle)).kind {
  67                             return;
  68                         }
  69                         let haystack = if let ExprKind::MethodCall(ref path, _, ref args) =
  70                                 filter_args[0].kind {
  71                             let p = path.ident.name;
  72                             if (p == sym!(iter) || p == sym!(iter_mut)) && args.len() == 1 {
  73                                 &args[0]
  74                             } else {
  75                                 &filter_args[0]
  76                             }
  77                         } else {
  78                             &filter_args[0]
  79                         };
  80                         let mut applicability = Applicability::MaybeIncorrect;
  81                         span_lint_and_sugg(
  82                             cx,
  83                             NAIVE_BYTECOUNT,
  84                             expr.span,
  85                             "You appear to be counting bytes the naive way",
  86                             "Consider using the bytecount crate",
  87                             format!("bytecount::count({}, {})",
  88                                     snippet_with_applicability(cx, haystack.span, "..", &mut applicability),
  89                                     snippet_with_applicability(cx, needle.span, "..", &mut applicability)),
  90                             applicability,
  91                         );
  92                     }
  93                 };
  94             }
  95         };
  96     }
  97 }
  98
  99 fn check_arg(name: Name, arg: Name, needle: &Expr<'_>) -> bool {
 100     name == arg && !contains_name(name, needle)
 101 }
 102
 103 fn get_path_name(expr: &Expr<'_>) -> Option<Name> {
 104     match expr.kind {
 105         ExprKind::Box(ref e) | ExprKind::AddrOf(BorrowKind::Ref, _, ref e) | ExprKind::Unary(UnOp::UnDeref, ref e) => {
 106             get_path_name(e)
 107         },
 108         ExprKind::Block(ref b, _) => {
 109             if b.stmts.is_empty() {
 110                 b.expr.as_ref().and_then(|p| get_path_name(p))
 111             } else {
 112                 None
 113             }
 114         },
 115         ExprKind::Path(ref qpath) => single_segment_path(qpath).map(|ps| ps.ident.name),
 116         _ => None,
 117     }
 118 }